The objectives of this project are to isolate and characterize a growth factor from serum which is responsible for enhanced mammary epithelial and other cell growth. We have been successful in isolating and partially purifying a factor, mammary serum factor, from porcine serum which is an acidic protein of molecular weight 10,400, which is capable of increasing the incorporation of tritiated thymidine into quiescent mammary epithelial cells by ten-fold and results in net DNA synthesis. We have demonstrated that mammary serum factor is not insulin, NGF, EGF, MSA, somatomedin A or C, or PDGF. We have developed radioimmunoasssay for this factor. We have also developed hybridomas which produce mono-specific antibody to MSF. We have found that lithium ions are also capable of initiating DNA synthesis in normal mammary epithelium, malignant mammary epithelial tumors, and in tissue culture cell lines when used over a concentration range of 1 to 15 mM. This effect is both on tritiated thymidine incorporation, cell number, and accumulation of DNA in cell culture. The effect is additive with insulin in normal epithelium and is synergistic in some physiological states.